This application is the national phase under 35 U.S.C. xc2xa7371 of PCT International Application No. PCT/GB97/03052 which has an International filing date of Nov. 6, 1997 which designated the United States of America.
The present invention relates to temperature sensors, sensing apparatus, and in particular, although not exclusively, to temperature sensors used as the radiation detecting elements in bolometers.
Temperature sensors having temperature-dependent properties which can be measured electrically are well known, and include resistors, semiconductor devices such as diodes, and thermocouples. For each of these examples, the temperature of the sensor is typically determined by measuring the magnitude of a voltage across or a current through the sensor. A disadvantage with these sensors is that any noise developed in, or picked up by the sensor or the cables connecting the sensor to electrical measuring apparatus therefore results in noise on the measured temperature.
It is also known to use capacitors as temperature sensors, where temperature changes lead to dimensional changes of the dielectric material separating the capacitor""s electrodes, resulting in changes in capacitance which can be measured using a capacitance bridge. However, measurement of this capacitance, and hence the temperature of the sensor, to sufficient accuracy may be hindered by the inherent inductance and capacitance of the cables connecting the sensor to the measuring apparatus. Furthermore, in certain applications, for example where the sensor is located in a harsh environment, it may not be possible to position the measuring apparatus close to the sensor in order to overcome this problem.
Bolometers incorporating temperature sensors are well known devices for measuring radiation, and in these devices the temperature sensors are arranged to have temperatures dependent on the flux of incident radiation.
Different types of bolometer are used for measuring different types of radiation in various environments; in fusion experiments, for example, bolometers are used to measure quantitatively the radiation and neutral particle emission from the plasma.
For such an application, a bolometer should preferably feature high signal to noise ratio and good resolution both in time and radiation flux, and comprise a temperature sensor having high temperature and vacuum compatibility, connected to the rest of the bolometer by a minimum number of cables. Additionally, the bolometer should be insensitive to fusion-specific noise sources, and the temperature sensor should be small in size, resistant to neutron and gamma-irradiation, and be insensitive to the presence of high magnetic fields.
The most commonly used bolometer in fusion research is based on a temperature sensitive thin metal resistor that forms part of a Wheatstone Bridge. Other resistance bolometer designs use semiconductors as their temperature sensors, because of their higher temperature effect. A further, different approach is a pyroelectric bolometer, where the spontaneous polarisation of a pyroelectric crystal is used as the temperature sensitive element.
The above designs have various disadvantages. The common problem of all, however, is that they are measuring absolute voltages or currents produced by the sensitive element, leading to considerable noise pick-up in electrically noisy environments. Future fusion devices will be larger, so that these signals have to be transmitted over longer distances, for example more than 100 m, making them even more susceptible to noise pick-up.
Other disadvantages arise from the neutron environment. The current designs are unlikely to be sufficiently radiation hard in future applications; resistance bolometers employ carrier foils, including mica and kapton, both of which contain hydrogen which may lead to rapid deterioration; semiconductor systems change their properties when irradiated with high neutron fluxes; and the electronics necessary for the pyroelectric bolometers may not be used close to the detector head.
In the field of infra-red detection, a bolometer is known in which a resistor is used as the temperature sensor, where the resistor forms part of an electronic active oscillator circuit. The frequency of oscillation of the circuit is dependent on the resistance of the resistor, which in turn is a function of its temperature. However, such a bolometer is unsuitable for use in fusion research, as the use of a resistive sensor again leads to considerable noise pick-up in a noisy environment, and the electronics necessary for the oscillator circuit may not be positioned in a region of high neutron flux.
Therefore, it is an object of embodiments of the present invention to provide a temperature sensor suitable for use in a noisy environment, and to provide a temperature sensor suitable for use in a bolometer for measuring radiation in fusion research.
It is an object of further embodiments of the present invention to provide sensing apparatus suitable for measuring temperature in a noisy environment.
It is an object of further embodiments of the present invention to provide a bolometer for measuring radiation in a noisy environment.
It is an object of further embodiments of the present invention to provide a bolometer for measuring radiation in fusion research.
It is an object of further embodiments to provide a bolometer sensor suitable for use in a noisy environment, and in particular for use in fusion research.
According to a first aspect of the present invention there is provided sensing apparatus comprising:
a temperature sensor comprising an inductor and a capacitor connected as a resonant circuit, wherein at least one of said capacitor and said inductor has a temperature-dependent reactance, whereby the resonant frequency of said resonant circuit is dependent on the temperature of at least one of said inductor and said capacitor; and
means for measuring the resonant frequency of said resonant circuit,
wherein said measuring means includes signal generating means responsive to said resonant circuit to generate a signal at the resonant frequency thereof and indicating means to provide an indication of the frequency of the signal generated by said signal generating means, and said measuring means in combination with said resonant circuit comprises a phase-locked loop (PLL) operable to track said resonant frequency.
According to a second aspect of the present invention there is provided sensing apparatus comprising:
a temperature sensor comprising an inductor and a capacitor connected as a resonant circuit, wherein at least one of said capacitor and said inductor has a temperature-dependent reactance, whereby the resonant frequency of said resonant circuit is dependent on the temperature of at least one of said inductor and said capacitor; and
means for measuring the resonant frequency of said resonant circuit,
wherein said measuring means includes signal generating means operable to generate a signal at a frequency lying within a frequency range and to scan said frequency across said range, said range including said resonant frequency.
According to a third aspect of the present invention there is provided sensing apparatus including:
a plurality of temperature sensors, each temperature sensor comprising an inductor and a capacitor connected as a resonant circuit, wherein at least one of said capacitor and said inductor has a temperature-dependent reactance, whereby the resonant frequency of said resonant circuit is dependent on the temperature of at least one of said inductor and said capacitor, each temperature sensor providing a respective different range of resonant frequencies;
means for measuring the resonant frequency of the resonant circuit of each of said sensors; and
a common transmission line,
said sensors being connected to said measuring means by said common transmission line.
An advantage of these three aspects of the present invention is that the resonant frequency of the resonant circuit is insensitive to noise, and hence the or each temperature sensor may be used in noisy environments.
A second advantage of these aspects of the present invention is that the resonant frequency of the circuit, being determined by the reactances of the capacitor and inductor, which in turn are determined by local conditions at the sensor, is insensitive to the characteristics of any attached cables, and hence may be measured accurately using measuring means connected to the resonant circuit by long cables.
Advantageously, the capacitor may have a dielectric whose permittivity is temperature-dependent, and this dielectric may be formed from ferroelectric material.
Conveniently, the capacitor may be a thin-film ferroelectric capacitor, formed as an integrated circuit device on a substrate.
Advantageously, the capacitor may be located on a region of the substrate having reduced, or minimal thickness, in order to improve thermal response.
The inductor may comprise a conducting loop, which may be formed for example as a planar thin film.
Advantageously, the sensing apparatus may be incorporated in a bolometer, and the capacitor may be arranged to have a temperature dependent on the flux of radiation incident on the sensor. Conveniently, the bolometer may be used in fusion research.
According to a fourth aspect of the present invention there is provided a bolometer sensor comprising:
two temperature sensors, each temperature sensor comprising a respective inductor and a respective capacitor connected as a resonant circuit, wherein at least one of said capacitor and said inductor has a temperature-dependent reactance, whereby the resonant frequency of said resonant circuit is dependent on the temperature of at least one of said inductor and said capacitor; and
a shield arranged to shield the temperature dependent inductor or capacitor of one of said sensors from at least a proportion of incident radiation of at least one type.
The two temperature sensors may be nominally identical, and may comprise ferroelectric capacitors formed on a common substrate. The difference between the resonant frequencies of the two circuits may then be used as an accurate indication of the intensity of radiation of the predetermined type incident on the sensor.
According to a fifth aspect of the present invention there is provided a temperature sensor comprising an inductor and a capacitor connected as a resonant circuit, wherein said capacitor is a ferroelectric capacitor formed by integrated circuit techniques on a substrate, whereby the resonant frequency of said resonant circuit is dependent on the temperature of said capacitor.